1. Field of the Invention
This invention relates to a boom crane and, more particularly, to a step for the boom of such a crane to prevent upward movement of the crane boom beyond a predetermined extreme upper position.
In boom cranes, the boom structure, which is usually pivotally mounted on the revolving superstructure of the machine for vertical swinging, is normally supported by wire ropes extending from a winch on the superstructure to the outer point of the boom, and the load is raised or lowered by a wire rope extending from a winch on the revolving superstructure over a sheave at the outer point of the boom. When such machines are in use, there is a danger that the boom will recoil, or swing upwardly past a vertical position and back over the frame, in the event the load is suddenly released, such as by the breaking of a wire rope.
Crane boom stops normally limit boom rotation by providing a physical connection between the revolving superstructure and the base section of the boom to limit the boom angle (angle between the horizontal and the boom centerline) to 90.degree. or less.
2. Description of the Prior Art
Of the devices presently used to limit boom rotation beyond a predetermined extreme upper position, the most basic is the "fixed" or "rigid" type boom stop. This usually consists of some structure mounted rigidly to either the boom or the revolving superstructure. The structure is contacted by the member (boom or superstructure) at the desired maximum boom angle.
Another type of boom stop commonly used is one composed of two tubular struts, each pivotally connected to the superstructure. Two link-type members pivotally connect the boom lower section to the extending ends of the struts. These links, following the booms vertical rotation, guide the strut ends toward target pads on the boom. A similar operating boom stop device can be made by mounting the tubular struts to the boom and the guiding links to the revolving superstructure. (See U.S. Pat. Nos. 2,999,601; 3,123,223; and 3,647,087).
The "rail" type boom stop is another device sometimes used to limit boom elevation. Two tubular struts extend from the revolving superstructure toward the boom. These struts are not connected to the boom when the boom is at low angles. As the boom is raised to a working angle, rollers on the ends of the struts engage rails fastened to the boom. The rollers ride on the rails as the boom rises and guide the strut ends to targets on the boom when the boom attains a maximum angle. An example of the rail type stop for a boom is shown in U.S. Pat. No. 3,092,261.
Still another boom stop device utilizes wire rope to stop boom elevation. One or more wire ropes are attached to the revolving superstructure and pass under the boom hinge to attach to the under side of the boom. As the boom reaches its highest angle, the wire ropes tighten and restrain the boom from further elevation.
Another type of boom stop consists of a pair of telescopic, strut-like members, which are pivotally connected to the revolving superstructure and the boom lower section. The strut-like members pivot vertically and telescope to allow the boom freedom to rotate in a vertical plane in the working range. The struts restrict the boom to some maximum angle by some means which stops the telescopic action at some preset maximum boom angle. The mechanism which incorporates the telescoping members has these advantages over the previously mentioned stop devices.
(1) With the telescopic device, the stopping force can be applied at a higher point on the boom than the other stop devices because of its telescopic ability to change length and follow boom elevation. The application of the stopping force at a higher point on the boom reduces stresses in the boom and stops when boom restraint is required. PA1 (2) Telescopic devices do not have free ends which can oscillate while the machine is working because they are pivotally connected to both the revolving superstructure and the boom at all times. PA1 (39 Telescopic devices utilize columnar members to carry the stopping force of the boom so that they may be sized to carry any load required. Wire ropes used in devices have a practical size limitation because they must be reasonably flexible.
Stop mechanism with telescopic members are shown in U.S. Pat. Nos. 2,479,838; 2,509,686; 2,627,985; 2,850,244; and 3,097,749.
Another boom stop device of interest is the one shown in U.S. Pat. No. 3,076,560. In this device two rods are pivotally connected to the boom. Each is received in a trunnion pivotally connected to the mast. A stop is fixed on the superstructure which is engaged by the rod when the boom reaches its extreme upper position.
Any or all of the previously mentioned stop devices may be equipped with some type of energy absorbing or energy dissipating mechanisms. The purpose of these mechanisms is to reduce the impact loadings encountered in stopping a recoiling boom. The energy absorbing devices commonly used include: spring, bumpers, and/or compressed air cushions.